core: more descriptive vendor image mismatch message

* On lineage, we don't update users' vendor images. This can
  cause an issue when the fingerprint of the rom doesn't match
  the fingerprint on the vendor image. Let's use a more descriptive
  message to tell the user what needs to be done, rather than telling
  them to contact their manufacturer.

Change-Id: I8ffd0fb7347454a52aa71498719a407447e6e8f4